PhD research aims to discover how painted turtles select their mates
BY ANDREW VOWLES
PhD candidate Elinor Hughes will do DNA analysis on painted turtle hatchlings next spring. Photo by Martin Schwalbe
Elinor Hughes hadn't planned to become a matchmaker for painted turtles living in Algonquin Park. But pairing up prospective turtle mates and filming the resultant action — or lack of it — is what this Guelph psychology graduate turned herpetologist is now doing for her doctoral studies in the Department of Integrative Biology.
Along with her supervisor, Prof. Ron Brooks, she is combining the tools of a video dating show with DNA technology familiar to TV forensics fans to learn more about how painted turtles choose mates. She's especially intrigued by a question that has puzzled biologists studying turtles in northern Ontario for almost three decades. Why do female turtles outnumber males three to one, and what's the effect of that sex ratio on mate selection among turtles living and courting in Algonquin Park's spruce bogs?
“Turtles do some interesting things,” says Hughes, who recalls Yertle the Turtle was one of her storybook favourites as a child. She cites everything from their hibernating abilities (hatchlings essentially freeze during winter, then thaw in spring) to their peculiar inability to swallow out of water (they need water pressure to do the trick). But it's their sex lives that really fascinate her.
In turtle populations where both sexes are balanced, females may look for physical characteristics in prospective mates, says Hughes, but females with limited options may end up pairing with just about any male that happens along. Skew the ratio so that males outnumber females, and you should start to see sexual selection intensify. Not only would females become more choosy, goes the notion, but the males would begin looking for ways to stand out in the crowd.
An obvious demonstration of sexual selection on land is the male peacock's tail, an attention-grabber that serves no other purpose than catching a courting female's eye. Among male painted turtles, sexual dimorphism shows up as longer foreclaws than in females.
In addition, males are generally about three-quarters the size of females. Hughes speculates that smaller males may be more nimble and less conspicuous than their larger counterparts, meaning they're better at evading predators in the pond — traits that would end up favouring smaller males and their offspring over time. Or maybe selection is working on the female: perhaps a larger female is simply able physically to produce more eggs.
At the Algonquin research station, Hughes has mixed and matched turtles of varying sizes and characteristics in hopes of filming the mating ritual and learning what females are looking for.
Setting four males and two females at a time into artificial ponds — actually tubs measuring about two metres across — she has tried to capture the action on video. It would be too difficult to do the work in real ponds, which are too large and murky for her purposes. Because the turtles seem easily distracted by her presence, she sets the video camera rolling, then discreetly leaves the turtles alone.
The results have been mixed. Hughes had hoped to get a good look at the male's courtship ritual, which includes a mixture of neck-stretching and nosing at the female's upper shell. It also involves waving those foreclaws before the female — a possible clue to why bigger claws might matter. But the only foreclaw action she saw turned out to be a male mistakenly displaying for another male.
She'd also hoped to observe female acceptance. “So far, the only female responses I've seen are running away and biting.”
Hughes plans to return with her video camera next summer.
Meanwhile, she's hoping to learn more by performing DNA analysis on about 270 hatchlings she brought back to U of G this year. The toonie-sized turtles — dark brown with characteristic red flashes marking the edge of their shells and limbs — are currently entering forced hibernation in vermiculite-filled plastic tubs in a climate-controlled room in the Hagen Aqualab.
There they'll stay until Hughes revives them in the spring for paternity testing, which involves taking tissue and blood samples. She'll compare the results with DNA collected from the moms and dads to see which youngsters belong to which mating pairs.
She's also intrigued about seeing how the turtles overwinter in sub-zero temperatures. Another researcher at Carleton University is studying hatchling freeze tolerance to learn more about possible applications to human cryogenics, she says.
Unlike in mammals, the sex of newly developing painted turtles is determined not by genetics but by environmental conditions. “Depending on the temperature, offspring are male or female,” says Hughes. “There's no genetic basis for sex.”
That means sex ratios may also be affected by short-term factors such as the mother's choice of a shady location for a nest or by larger issues. Another of Brooks's PhD students, Sarah Holt, is studying snapping turtles — a species that spans most of North America — to see whether temperatures at different latitudes affect their gender.
As for long-term climate change, Hughes points out that turtles shared the Earth with the dinosaurs during the Triassic period. “They've been through a heck of a lot. With all the other known pressures, what if they're compounded with a rapid temperature shift?”
Brooks has studied turtles in northern climates for about 25 years.
“It's clear that climate change is well on the way and that there will be huge consequences,” he says. “But even though it seems logical that warming climate could lead to turtles becoming all one sex and then disappearing, they have survived previous climate change better than almost any other group and still retained temperature-determined sex. How do they do it?”
